add HOMEBREW_TAP_FORMULA_REGEX constant in tests

because our code tries to use it when throwing exceptions
also move HOMEBREW_BREW_FILE from doctor_test.rb to test_helper.rb

test/cask/cli/doctor_test.rb

@@ -1,8 +1,6 @@
 require 'test_helper'
 require 'cask/version'
 
-HOMEBREW_BREW_FILE = '/usr/local/bin/brew'
-
 describe Cask::CLI::Doctor do
   it 'displays some nice info about the environment' do
     out, err = capture_io do

test/test_helper.rb

@@ -1,6 +1,9 @@
 require 'bundler'
 require 'bundler/setup'
 
+# set some Homebrew constants used in our code
+HOMEBREW_TAP_FORMULA_REGEX = %r{^(\w+)/(\w+)/([^/]+)$}
+HOMEBREW_BREW_FILE = '/usr/local/bin/brew'
 
 # add cask lib to load path
 brew_cask_path = Pathname.new(File.expand_path(__FILE__+'/../../'))